dormer window insulation

Hi

There are dormer windows on my father's house, which was originally a bungalow.

The construction was 'traditional' with rafters running from steel beams near the ridge to the window line. Chipboard decking was then fitted and 2 layers of felt stuck on top. Before the builder put on the fascia I actually squashed in fibreglass insulation into the gaps, trying to get as much in as I could. I worked on the principle : the more insulation the better.

Well, that was about 20 years ago and the roof has now started to leak. On inspection, the board in 2 spots seems spongy, so obviously it is time for the full replacement.

Not a cheap job, but worthy I am sure

On checking web sites, it seems that you are supposed to have a 50mm gap on top of the insulation, for ventilation to prevent condensation.

Now to be perfectly honest, this was not done before and there has never been a condensation problem so far.

Any advice would be appreciated, as obviously I want the job to be done right, coupled to it being economically the best option, with regards to insulation.

If jamming ther insulation in worked before, surely it would work again.